Certain web properties lend themselves to optical inspection. Such properties may either be directly observable (such as transmissivity, or aberrations such as scratches or other cosmetic defects) or be sufficiently correlated to an optically-observable property as to be measurable. For example, for a non-woven web, a property correlated with an optically-observable property, but not directly observable, is its insulating capability, which is often measured by thermal conductivity. A web's thermal conductivity can be measured by monitoring the rate of heat flux across a known temperature gradient, but such measurement is difficult in on-line, production-type environments. However, if the web's construction allows for light transmission, its thermal conductivity may be correlated to the brightness of light transmitted through it and into a series of optical sensors. Thus, using the optical signal of light transmitted through the web, the web's thermal conductivity can be characterized using known optical sensing technologies, provided the optical signal can be calibrated to thermal conductivity units. Thermal conductivity is just one example of a property that may be measured by optical inspection techniques. Other example properties include surface roughness, thermal diffusivity, porosity, crystallinity, and thickness, to name a few, as well as of course optical properties like optical density, transmissivity, reflectivity, and birefringence.
An issue for quantitative inspection schemes (and to a lesser degree qualitative inspection schemes) is the calibration of the sensor or sensors that comprise the web inspection system such that it yields data about the web property of interest in units of measurement calibrated to be within some margin of error of a known standard.